Maintaining the security and physical integrity of undersea pipelines is vital to the economic and environmental stability of the modern world. The sheer length of undersea pipeline is vast, e.g., approximately 30,000 miles of pipeline exist on the Gulf of Mexico seafloor alone. These lengths present a formidable challenge to any undersea monitoring system requiring persistence. Because of the potential severity of an undersea pipeline rupture, the detection, localization, and timely prosecution of an intrusion are critical. Intrusion is here defined as the introduction of an undesired material element that is normally not, present within a radial proximity to the pipeline or an incipient mechanical vibration of material elements normally present within the proximity. Intrusions include, for example, undersea vehicles (manned or unmanned remotely operated or autonomous), underwater robots, divers, diver delivery vehicles, directed or intense sound waves, or leaking fluid from within the pipeline entering the exterior proximity. A need therefore exists for a persistent monitoring system able to detect objects or disturbances in proximity to the pipeline.
Traditional means for monitoring the proximity of an undersea pipeline typically involve mounting a relatively high-frequency acoustic sonar “search light” on or near the pipeline at regular intervals or monitoring by undersea vehicles. These alternative approaches, while possibly adequate over short pipe lengths or restricted regions, become logistically or financially prohibitive over time. Systems that operate at 60 kHz, for example, require mounting an acoustic source every 1 or 2 km. Besides the installation expense, this permanent installation of many sources incurs practical difficulties due to fouling of the hardware and the expense of servicing the equipment. Moreover, it may be impractical to install a pipe-mounted or pile-moored acoustic source in deep water in a given situation. Communications, telemetry or power equipment associated with this conventional sonar approach may also be visible above the water line, which may be undesirable depending on a given situation.